- Group index of soil is a number used to refer the quality of soil used as a subgrade material in highway construction.
- Group index of soil is also used in AASHTO classification of soil. In AASHTO soil classification, group index is also included along with soil group and subgroup.
- Group index of soil as a subgrade material is calculated using liquid limit, plastic limit, plasticity index and % passing from US sieve #200 (i.e, 0.075 mm opening sieve).

- So, group index of soil can be defined as the combination of :
- Partial group index obtained from liquid limit.
- Partial group index obtained from plasticity index.

### Group index of soil Formula

GI = (F₂₀₀-35) [0.2+0.005(LL-40)] + 0.01(F₂₀₀-15)(PI-10)

Here, (F₂₀₀ -35) [0.2+0.005(LL-40)] = partial group index obtained from liquid limit

0.01(F₂₀₀ -15) (PI-10) = partial group index obtained from plasticity index

Note : In short form, the GI formula can also be written as

GI = 0.2a + 0.005ac + 0.01bd

here, a = (% passing # 200 sieve)-35

b = (%passing #200 sieve)-15

c =(LL-40)

d = (PI-10)

- Group index of soil is always rounded off to nearest whole number. If group index of soil comes to be negative , then group index of soil in this case is considered zero.
- The value of group index of soil ranges from 0 to 20. A soil having lower group index value is considered better subgrade material in comparison to that of soil having higher group index value.

- In AASHTO soil classification system, the group index value for
__Coarse grained soil__- A-1-a, A-1-b, A-2-4, A-2-5, A-3, GI= 0
- A-2-6 and A-2-7 , GI = 0.01(F₂₀₀-15)(PI-10) ,i.e, partial group index obtained from plasticity index.
__Fine grained soil__- GI = (F₂₀₀-35)[0.2+0.005(LL-40)] + 0.01(F₂₀₀-15)(PI-10)

Note : For coarse grained soil, the partial group index obtained from liquid limit is neglected.